I've been expecting it to be extended up to 256 for some time now, but
it hasn't happened yet.

When IBM was designing System/360 there was a proposal for an ASCII-8
that was not just an extension with the ASCII-7 characters as the
first 128. With bits numbered from 0 (lsb) to 7 (msb) ASCII-8
would have been formed by duplicating bit 5 as bit 7 for the
ASCII-7 characters. There were some features of the S/360 instructions
designed around that, but were removed in designing S/370.
(The bit to enable them required supervisor mode, and no OS were
known to activate that bit.)